1. Field of the Invention
The present invention relates to a process and an apparatus for suspension polymerization. More particularly, it relates to a process and an apparatus for suspension polymerization which can produce polymer particles having a uniform particle size.
2. Description of the Related Art
As a process for suspension polymerization which can produce polymer particles having a uniform particle size, is known a suspension polymerization process which comprises steps of suspending a monomer liquid in an aqueous dispersion medium to form monomer liquid droplets in a column type reactor and then polymerizing the monomer without applying strong shear to the monomer liquid droplets, whereby the monomer can be polymerized with suppressing merging or splitting of the monomer liquid droplets.
For example, Japanese Patent Publication No. 36510/1978 or U.S. Pat. No. 3,922,255 discloses a suspension polymerization process in which a monomer liquid is passed through an orifice to form droplets having a rather small and uniform size in an aqueous dispersion medium. The droplets are charged in a column type reactor, there provided with a velocity toward a bottom of the reactor by a downflow of the aqueous dispersion medium, and the monomer liquid is partially polymerized in the reactor to such extent that the droplets are not merged or split with gentle shear. Then, the partially polymerized monomer liquid droplets are discharged from the bottom of the reactor together with the aqueous dispersion medium and introduced in a solid-liquid separator which is connected with the reactor. In the separator, the partially polymerized droplets are floatation separated and concentrated by difference of densities between the droplets and the dispersion medium. The separated droplets are charged in an agitated reactor and suspension polymerized with avoiding merging or splitting to give polymer particles with a considerably uniform size.
Japanese Patent Kokai Publication No. 91701/1983 or U.S. Pat. No. 4,487,898 discloses a suspension polymerization process in which a laminar flow of a monomer liquid is gushed through an orifice in an aqueous dispersion medium to form monomer droplets having a uniform size, and the droplets are charged in a column type reactor and polymerized under a fluidized condition to such extent that specific gravity of the monomer liquid droplets becomes close to that of the aqueous medium. A slurry comprising the partially polymerized monomer liquid droplets and the dispersion medium is circulated through the tower type reactor and a settling separator, in which the partially polymerized monomer liquid droplets having specific gravity larger than that of the dispersion medium are separated and concentrated, and then charged in a second column type reactor to further continue suspension polymerization, whereby polymer particles having a considerably uniform size are continuously produced.
Characteristics of the above processes are as follows:
The monomer liquid droplets having a considerably uniform size are formed with the use of an orifice. The monomer liquid droplets having the uniform size distribution are kept in a column type reactor with forming suspension condition by a flow of an aqueous dispersion medium for a preselected period to form polymer particles having the uniform size.
Thus, selection of a suspension and protective agent is particularly important since suspension condition of droplets should be kept by only a flowing of an aqueous dispersion medium in a reactor.
Although Japanese Patent Publication No. 36510/1978 teaches that a conventional suspension and protective agent can be used, the concrete agent disclosed in examples is sodium polyacrylate or a polymer protective colloid such as an animal protein gel buffered with sodium borate in pH range of 6 to 12.
Although Japanese Patent Kokai Publication No. 91701/1983 also teaches that a generally used suspension and protective agent can be used, actually claimed one is a combination of an organic protective polymer colloid with an aqueous polymerization inhibitor, or a water-insoluble inorganic material.
Japanese Patent Kokai Publication No. 102905/1982 discloses a process for suspension polymerization in which a monomer liquid is excited under vibration when the monomer liquid is gushed in an aqueous dispersion medium to form droplets having a uniform size, and then polymerized under condition where the droplets are not merged or split. Said Publication concretely describes only that an organic polymer material or a water-insoluble inorganic material is used as the protection agents for such condition.
On suspension polymerization in the column type reactor using the above process, a suspension and protective agent mainly comprising the polymer protective colloid should be used in such an amount that the monomer liquid droplets are not substantially merged or split since the suspended condition of the droplets has to be maintained by the function of an aqueous dispersion medium which forms the downflow in the column type reactor.
However, when the polymer protective colloid is used as the suspension and protective agent, the aqueous dispersion medium which is separated from the polymer particles is discharged as a waste water from the process, and the waste water should be treated with a higher order treatment of waste water in order to avoid water pollution.
In order to avoid the above problem, a conventional suspension and protective agent mainly comprising a water-insoluble inorganic material in the form of fine powder is used instead of the polymer protective colloid. However, in a column type reactor which applies only gentle shear to the medium, the water-insoluble inorganic material constituting a main component in the aqueous suspension agent is completely adsorbed on the monomer liquid droplets, whereby a concentration of the water-insoluble inorganic material in the aqueous dispersion medium is greatly reduced. When such an aqueous dispersion medium is recycled in the reactor, the concentration of the inorganic material is too small to protect the monomer liquid droplets which are newly charged in the reactor.